1. Field of the Invention
This invention pertains generally to electrical switching apparatus and, more particularly, to ground fault circuit breakers. The invention also relates to methods of detecting a ground fault.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. For example, in response to the overcurrent condition, a spring powered operating mechanism is unlatched, in order to open the separable contacts of the circuit breaker and, thus, interrupt current flow in a protected power system. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,910,760; 6,137,386; 6,144,271; and 6,853,279, which are incorporated by reference herein.
In many applications, the circuit breaker also provides ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. For example, this trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism to trip open the separable contacts.
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. Nos. 5,260,676; and 5,293,522.
Ground fault current is conventionally sensed or measured with some sort of summing current transformer. For a three-conductor (e.g., phases A, B and C) or four-conductor (e.g., neutral plus phases A, B and C) power circuit, for example, this current transformer sums the three or four conductor currents and provides an output that corresponds to the amount of imbalance between the conductor currents.
On some known plural-pole circuit breakers, the ground current is sensed either internally via a secondary current transformer (e.g., mounted on a printed circuit board to sum the secondary currents out of the various phase current transformers) or externally via a primary current transformer (e.g., a source ground current transformer, which sums the primary current). In either example, the output of the current transformer, which should normally be zero, represents any ground fault current.
There is room for improvement in electrical switching apparatus that provide ground fault detection.
There is also room for improvement in methods of detecting a ground fault.